Conventionally, a photocationic polymerizable adhesive containing an epoxy resin as a main component has been used as a kind of adhesive used for mounting an electronic part such as an IC chip on a wiring board. Such a photocationic polymerizable adhesive has been formulated with a photocationic polymerization initiator that initiates cationic polymerization by generating protons via light. A sulfonium antimonate complex is known as such a photocationic polymerization initiator.
A sulfonium antimonate complex, however, has as a counter anion SbF6− having fluorine atoms bonded to a metallic antimony, thereby causing a problem of producing large amounts of fluorine ions during cationic polymerization to corrode a metal wiring and a connection pad. Thus, it has been suggested that a sulfonium borate complex with a tetrakis(pentafluorophenyl)borate anion [(C6F5)4B−] having fluorine atoms bonded to carbon atoms instead of SbF6− is used as a cationic polymerization initiator (Patent Document 1). In fact, the complex, [p-hydroxyphenyl-benzyl-methylsulfonium tetrakis(pentafluorophenyl)borate], of the following formula (1c) is commercially available.

Actually, a joining portion cannot be irradiated with light in many cases when an electronic part is mounted on a wiring board. Thus, an attempt has been made to divert the specific sulfonium borate complex disclosed in Examples of Patent Document 1 to a thermal cationic polymerization initiator for a thermal cationic polymerizable adhesive. In such a case, it is desirable that the amount of fluorine ions generated during cationic polymerization be reduced to improve not only electrolytic corrosion resistance but also low-temperature fast curing properties of a thermal cationic polymerizable adhesive for productivity improvement.